Linker error, using g++ to link to a library compiled by gcc, undefined reference to that function

Question

I wrote a C function which calls a function defined in openssl/sha.h , let's say the declaration is "helper.h" and implementation is "helper.c ". Then I compiled that function using gcc and put it in a library:

gcc -std=c99 -lcrypto -lssl -c helper.c -o helper.o
ar -rc helper.a helper.o

Then I wrote a cpp function that calls the function declared in "helper.h" . I added

extern "C" {
#include "helper.h"
} 

stuff in that cpp file and then I used

g++ test.cpp helper.a -o cpp

Then the ld error occurs and says I encounter undefined reference for a function defined in openssl/sha.h . I wonder how to resolve that?

Answer 1

Add ssl and crypto libs to your final command in order to link these libs to your program.

You must also respect the order: since sll is using crypto so you must put ssl first and crypto after.

g++ test.cpp helper.a -o cpp -lssl -lcrypto